In what a promotional ad calls "the most shocking investigation in Dr. Oz Show history," Oz points to tests commissioned at a private lab. Those tests found high levels of total arsenic in one well-known brand of apple juice.

Tipped off by the show, the FDA immediately tested the same lots of apple juice. Their tests showed considerably lower amounts of total arsenic.

But total arsenic isn't the point, the FDA says. Only inorganic arsenic is poisonous to humans. More sensitive tests for the harmful form of arsenic found only trace amounts of the poisonous substance.

What does this mean? In the promo for his show, Oz speaks passionately. "How can I be standing here in America talking about arsenic in our apple juice?" he asks. "What the heck is going on?"

Acceptable Levels of Arsenic

The Environmental Protection Agency (EPA) sets a standard of 10 parts per billion (ppb) of arsenic in drinking water. But that's for "long-term, chronic exposure to arsenic in drinking water," according to the EPA. According to the U.S. Agency for Toxic Substances and Disease Registry, drinking water generally contains about 2 ppb of arsenic, although some areas have considerably higher levels.

The FDA spot checks juice products and automatically does further tests on any juice containing 23 ppb or more of arsenic.

"But even if it comes in lower, we have a right to test that product, too," FDA spokeswoman Stephanie Yao tells WebMD. "This is not a new issue to us. We have been monitoring products for arsenic for years now. And if high levels of inorganic arsenic are in a product, that is an FDA concern."

Yao says that any juice product with 23 ppb of inorganic arsenic "is a concern."

In the past, she says, the FDA has alerted consumers to high levels of arsenic in grape juice from Argentina and in pear juice from China.

Despite the Dr. Oz Show investigation, the FDA is not raising the alarm about any brand of apple juice. Instead, the agency is reassuring U.S. consumers that their apple juice is safe.

The U.S. Apple Association points to the FDA's Total Diet Study of 2006 to 2008. That document shows that bottled apple juice contains an average of 5 ppb of total arsenic, and that very few samples test higher than 13 ppb.

"USApple supports responsible, research-based approaches for ensuring the safety of apples and apple products and maintaining consumer confidence in them," the industry trade group says in a statement released in response to the Dr. Oz Show.